1. Use a 1-quart fancy mold, small loaf pan or ring mold. Chill in the refrigerator until the gelatin mixture is ready. Then rinse the mold but do not wipe it.
2. Dissolve the gelatin in the boiling water. Let it cool.
3. Add the lemon or lime juice, grated peel, horseradish and onion. Chill.
4. When the liquid begins to set, add the mayonnaise, salt and chopped avocado. Mix well. Pour into the rinsed mold.
5. Chill until firm, 1 hour or longer. Unmold, slice and serve on lettuce with French Dressing.